1. Field of the Invention
The present invention relates generally to an apparatus and method for internally coating live gas pipeline or conduit. Particularly, this invention relates to internally coating pipe joints or other discontinuities in live propane or natural gas pipeline. Even more particularly, this invention relates to internally coating pipe joints or other discontinuities in live propane or natural gas pipeline using a self-aligning, explosion-proof coating device capable of coating several hundred feet of live underground gas pipeline while requiring only a single excavation.
2. Description of the Related Art
A dilemma has arisen in the gas and gas transportation industry. Low-moisture gas, such as propane or natural gas, has replaced high-moisture manufactured gas, such as coal gas, as a source of domestic and industrial fuel. Traditionally and for many decades coal or other high-moisture gases were fed to customers by underground pipes. Typically these gas pipelines were constructed of individual lengths of pig or cast iron pipe. These individual lengths of pipe were commonly joined together by bell or lap joints that were sealed with a combination of a filler material and lead. Several different types of filler material were used including horsehair, yarn, jute and hemp. It was discovered that, as many municipalities converted from high-moisture manufactured gas to the relatively low-moisture propane or natural gas, the filler material in the pipe joints would dry out. As these filler materials dried out they would decompose and disintegrate, thereby causing gas leaks to appear at the pipe joints.
The decay of joint filler due to the conversion to low-moisture gas is not unique to the United States. The United Kingdom is experiencing similar decay of their gas pipe joint filler. As a preventative measure, and as an attempt to slow down the decay of filler material, many gas companies in the United Kingdom, and a few in the United States, routinely "fog" their gas lines. Fogging normally involves sending a glycol type product through the gas pipeline to enhance the moisture content of the filler. Another method of maintaining high moisture in the filler involves a process known in the gas industry as humidification. This process requires repeated application of pressurized steam to a gas pipe system.
Unfortunately, these preventative procedures are only temporary and can be quite costly. Today, to adequately prevent gas from escaping these types of pipelines, the pipe joints or other discontinuities must be sealed or replaced. Because many of these pipelines are underground and not readily accessible, excavating, removing and replacing an entire length of pipeline having deteriorated pipe joints is drastic and quite costly.
One method of sealing these pipe joints or discontinuities against gas leaks includes excavating an end of the pipe and having someone climb into the pipe to hand apply a coating compound. This method can be quite expensive and time consuming. Also, this method can be dangerous and is unfeasible for small diameter pipe. Another technique includes inserting a permanent lining throughout the entire length of pipe. Again this is quite costly and may cause an unacceptable reduction in the flow capacity of the pipe. Also, this method requires a large consumption of natural resources to fabricate a lining for an entire length of pipe, when typically only the joints are susceptible to leaking.
Still another method, such as U.S. Pat. No. 4,178,875 (1979, Moschetti) includes sending a device through the pipe that can remotely detect a joint or other discontinuity that needs repair. A coating material is then sent through attached tubing and is sprayed onto the inner surface of the pipe at the desired location. However, this and the above-mentioned methods are not performed on "live gas pipe" (pipe in which pressurized gas remains flowing). These methods require the gas flow to be shut down for long periods of time. Depending on the customers being serviced by the gas line, it is normally unacceptable to interrupt service for such long periods of time. Another disadvantage of these methods is that they require more than a single excavation when coating long lengths of pipeline.
Still other methods are known whereby the gas remains live while coating, repairing or sealing is accomplished. U.S. Pat. Nos. 4,582,551 and 4,627,471 (1986, Parkes et al.) disclose a method and device that can remotely seal joints or leaks in a pipe while the gas continues flowing in the pipe. The device is inserted into a pipe whose inner diameter is slightly larger than the outer circumference of the device. The device uses expandable bladders to form a substantially air-free environment, thereby isolating the joint or discontinuity from pressurized gas. The pressurized gas is rerouted through the interior of the device. Anaerobic sealant is then pumped to the device and the sealant is sprayed onto the interior of the pipe at the desired location. The device remains in place long enough to allow the anaerobic sealant to setup. A disadvantage with this device is that it requires an environment free from air and flowing pressurized gas in which to apply sealant. Another disadvantage with these types of devices is that they are limited in their ability to maneuver around corners or other obstacles in the pipeline as they are in close proximity to the interior of the pipe. Still another disadvantage with these devices is that they are slow and time consuming because they require the device to remain in place while the sealant sets.
Another method of sealing pipe joints in a live gas pipe is taught in U.S. Pat. No. 5,156,886 (1992, Kitson). This method involves inserting a nozzle attached to a hose through a tapping mandrel to a desired location in a live gas pipe, whereby an anaerobic sealant is pumped through the hose to the nozzle. The nozzle sprays the anaerobic sealant onto the interior of the pipe. This method works well on relatively short lengths of pipe. However, as the length of tubing increases, the viscosity of the anaerobic sealant prevents it from reaching the spraying device in adequate quantities. Also, as the length of tubing increases, static electric charges build up in the line due to the friction caused by the sealant rubbing against the interior of the tubing. This can pose serious problems when working in a live gas setting. Another drawback with this device is that the anaerobic sealant tends to pool in the bottom of the pipe upon application. An additional drawback of this method is that it typically requires the presence of some filler to properly seal a leaking joint. Because the above-mentioned preventative or fogging measures were never routinely performed in the United States, much of the filler in United States gas pipe joints has disintegrated, making this method of sealing pipe joints impractical.
What is needed is an apparatus and method for coating or sealing the interior of very long lengths of underground pipe or other conduit, either at a joint, another type of discontinuity or along the entire length of the pipe, while the gas in the pipe remains live. What is also needed is an apparatus and method of internally coating live gas pipe joints that do not require the presence of filler material in a joint to efficiently seal it. What is further needed is an apparatus and method that allow for several layers of coating material to be applied at the same site, that provide a rapid coating process, that do not require the coating device to remain in place while the coating material sets, that can be inserted into a live gas pipe using a variety of means, that can maneuver around bends and other obstacles normally found in gas pipe, and that can be safely operable in live gas settings. Finally, what is needed is an apparatus and method that applies an epoxy-type sealant that will coat the interior of a variety of different pipe surfaces, with or without the presence of pressurized gas, with or without the presence of air or oxygen and that will not produce significant pooling of the coating material in the bottom of the pipe upon application.